This invention relates to a magnetoresistance material, viz. a conductive material that exhibits magnetoresistance, which is an inhomogeneous system consisting of a nonmagnetic matrix and ultrafine particles of a ferromagnetic material dispersed in the nonmagnetic matrix. Herein, the term "nonmagnetic" is in the sense of non-ferromagnetic and is inclusive of paramagnetic and diamagnetic. The magnetoresistance material can be formed as a film on a substrate and is useful, for example, in magnetic heads for writing/erasing information.
It is well known that magnetoresistance occurs in some magnetic multilayer systems consisting of at least one ferromagnetic layer, e.g. Co layer, and at least one nonmagnetic layer, e.g. Cu layer.
Besides, recent reports have disclosed magnetoresistance in nonlayered inhomogeneous systems in which ultrafine particles of a ferromagnetic material are dispersed in a nonmagnetic matrix: Berkowitz et al., Physical Review Letters, Vol. 68 (1992), pp. 3745-3748, and Xiao etal., ibid., pp. 3749-3752. Such inhomogeneous systems are sometimes called magnetic granular systems. Usually the inhomogeneous granular systems are in the form of films. For such systems various combinations of ferromagnetic and nonmagnetic materials have already been reported; such as Co/Cu, Fe/Ag, Co/Ag, Co/Au, Co--Fe/Ag, Ni--Co/Ag, Ni--Fe/Cu, Fe/Mg, Co--Fe/Cu, Ni--Fe/Ag, Ni--Fe--Co/Ag, etc.
A problem in the inhomogeneous granular systems is that the magnitude of magnetoresistance decreases when the films are long kept in the atmosphere. Herein, the magnitude of magnetoresistance will be referred to as MR change which is given by (R.sub.max -R.sub.min)/R.sub.min .times.100 (%), where R.sub.max and R.sub.min are the maximum and minimum resistances measured by varying the applied magnetic field. There are three possible reasons for the aging decrease in MR change.
First, there is a possibility that an alloy forms at the interfaces between the magnetic particles and the nonmagnetic material by interfacial diffusion. If an alloy formes at the interfaces the interfacial scattering of electrons becomes indistinct, and differences in the degree of the interfacial scattering with the directions of magnetization in the magnetic particles decrease, and consequently MR change decreases.
Another possible reason is oxidation of the nonmagnetic matrix. Partial oxidation of the nonmagnetic matrix causes an increase in the electron scattering in the nonmagnetic matrix and hence induces a relative decrease in the interfacial scattering. Also in this case the dependence of the interfacial scattering on the directions of magnetization of the magnetic particles reduces, and consequently MR change decreases.
The third possible reason is oxidation of the magnetic particles mostly in the particle surface regions. Since metal oxides are very high in electrical resistance, the metal oxide on the particle surfaces constitutes a serious obstacle to the interfacial scattering of electrons, and therefore MR change decreases.
Probably the above three possible phenomena actually occur altogether.